Yes. When you can replace the car for cheaper than you can repair the problem, you should simply replace the car. That's how a car gets "totaled." That is, it's a total loss, not because it's impossible to repair, but because it's not practical to do so. Is repairing the rear-end and any other problems you currently have more expensive than replacing your car with another similar car?
Also, if you were going for practicality, what business have you in a built mustang? You will replace a lot of parts. I too have built mustangs. My solution? I drive 165 miles a day round-trip to work. They go on a car that I've spent less on than any other in my life. That's what beaters exist for. Consider getting an economical, practical car you can depend on in addition to your project. The most practical of cars will set you back $1,500-$3k, will get 30-45 mpg, cost next to nothing for liability-only insurance & a multi-car discount, be more dependable than your mustang, be plentiful in junkyards for spare OEM parts, and depreciate little to nothing regardless of the mileage & years put on them.